Method and apparatus for semiconductor wafer cleaning using high-frequency acoustic energy with supercritical fluid

ABSTRACT

An apparatus and a method is provided for using high-frequency acoustic energy with a supercritical fluid to perform a semiconductor wafer (“wafer”) cleaning process. High-frequency acoustic energy is applied to the supercritical fluid to impart energy to particulate contamination present on the wafer surface. Energy imparted to particulate contamination via the high-frequency acoustic energy and supercritical fluid is used to dislodge and remove the particulate contamination from the wafer. Additionally, the wafer cleaning process benefits from the supercritical fluid properties of near zero surface tension, high diffusivity, high density, and chemical mixing capability.

CLAIM OF PRIORITY

This is a divisional application (under 35 U.S.C. 120) of U.S. patentapplication Ser. No. 10/357,664, filed on Feb. 3, 2003 now U.S. Pat. No.7,191,787. The disclosure of the above-identified application isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to semiconductor wafer cleaning.More specifically, the present invention relates to a method andapparatus for using high-frequency acoustic energy with supercriticalfluid to perform a semiconductor wafer cleaning operation.

2. Description of the Related Art

In the manufacture of semiconductor devices, a surface of asemiconductor wafer (“wafer” or “substrate”) must be cleaned to removechemical and particulate contamination. If the contamination is notremoved, semiconductor devices on the wafer may perform poorly or becomedefective. Particulate contamination generally consists of tiny bits ofdistinctly defined material having an affinity to adhere to the surfaceof the wafer. Examples of particulate contamination can include organicand inorganic residues, such as silicon dust, silica, slurry residue,polymeric residue, metal flakes, atmospheric dust, plastic particles,and silicate particles, among others.

Traditionally, wet-cleaning of a wafer has been performed usingconventional solvents composed of aqueous, semi-aqueous, or organicsolvent chemistries. In general, the conventional solvents can beapplied to the wafer in the form of a bath or rinse. Some wafer cleaningprocesses also incorporate mechanical assistance from scrubbing brushesor high-pressure sprays. Also, most wet-cleaning processes are followedby a deionized water rinse and subsequent wafer drying process.Depending on the solvent used, the properties of both the solvents andthe rinses used in the wet-cleaning process have a surface tensionproperty with a wetting angle that is a function of the surfacecharacteristics of the substrate. The surface may be hydrophilic,hydrophobic, or have properties somewhere in-between hydrophobic orhydrophilic. In cases where the surface is hydrophilic, a solution witha low wetting angle will easily wet the surface, and the fluid will bedrawn into high-aspect ratio features by capillary forces. Thesecapillary forces must be overcome to remove the liquid from the featuresafter cleaning. Therefore, the high surface tension causes the liquidsolutions to collect and adhere within structures present on the wafersurface, thus presenting difficulty during the drying process.

FIG. 1A is an illustration showing the collection and adherence of anaqueous or semi-aqueous solution 105 between high aspect ratio waferstructures 103 present on the surface of a wafer 101 following awet-cleaning process, in accordance with the prior art. High surfacetension causes the aqueous solution 105 to resist removal from betweenthe structures 103 during the drying process. Since the aqueous orsemi-aqueous solution 105 tends to be retained between the structures103, removal of the solution 105 can cause the collapse of very smallstructures due to the capillary forces. Hence, there is a potential forthe structures 103 to be damaged. This phenomenon is a well known issuein the cleaning and drying of MEMs structures, especially in MR heads.

FIG. 1B is an illustration showing distortion damage of the structures103 caused by capillary force collapse during the drying process, inaccordance with the prior art. As the aqueous or semi-aqueous solution105 is removed by a high speed spin, evaporation, or other means, thehigh aspect ratio wafer structures 103 can be forced together asindicated by arrow 107 due to the surface tension caused by thecapillary forces present. The distortion of the structures 103 canadversely affect subsequent wafer processing and ultimate deviceperformance.

FIG. 1C is an illustration showing delamination damage of the structures103 caused by capillary force collapse during the drying process, inaccordance with the prior art. Again, as the solution 105 is removedfrom the high aspect ratio features, the capillary forces can causecollapse of the high aspect ratio wafer structures 103 to the point ofdelamination from an underlying substrate material as indicated byarrows 109. The propensity of the wafer structures 103 to delaminate isa function of the bond strength between the structures 103 and theunderlying substrate material. Delamination damage of the structures 103will certainly cause subsequent wafer processing and ultimate deviceperformance to be adversely affected.

Aqueous or semi-aqueous solutions used for conventional solvents andrinses can also introduce difficulty through absorption into wafersurface materials. For example, aqueous solutions can be absorbed into aporous matrix, such as that of a porous low-K material. Driving offabsorbed aqueous solution from the porous matrix of the low-K materialduring the drying process can cause physical damage or changes to thelow-K material structure or enhance diffusion of contaminants throughthe low-K material. Physical damage, changes, and contamination of thelow-K material can degrade its performance. However, allowing theabsorbed aqueous or semi-aqueous solution to remain in the porous matrixof the low-K material can lower the dielectric constant of the low-Kmaterial and adversely impact device performance. Due to the difficultyassociated with the aqueous or semi-aqueous nature of conventionalsolvents and rinses, it is desirable to develop an alternative approachfor performing the wafer cleaning and rinsing process.

FIG. 2 is an illustration showing a generalized material phase diagram,in accordance with the prior art. The phase of the material isrepresented as regions of solid, liquid, and gas, wherein the presenceof a particular phase is dependent on pressure and temperature. Thegas-liquid phase boundary follows an increase in both pressure andtemperature up to a point called the critical point. The critical pointis delineated by a critical pressure (P_(c)) and a critical temperature(T_(c)). At pressures and temperatures beyond P_(c) and T_(c), thematerial becomes a supercritical fluid.

The supercritical fluid shares the properties of both a gas phase and aliquid phase. The supercritical fluid has near zero surface tension.Therefore, the supercritical fluid can reach into and between smallfeatures on the wafer surface without causing the problems associatedwith the high surface tension of an aqueous or semi-aqueous solution.Also, the supercritical fluid has a diffusivity property similar to agas. Therefore, the supercritical fluid can get into porous regions ofwafer materials, such as low-K dielectric material, without becomingtrapped. Additionally, the supercritical fluid has a density similar toa liquid. Therefore, more supercritical fluid can be transported to thewafer in a given amount of time as compared to a gas.

One prior art approach to using supercritical fluid in a wafer cleaningprocess is to fill a chamber with supercritical fluid and allow a waferto soak in the supercritical fluid. However, simply filling the chamberwith supercritical fluid and allowing the wafer to soak is notsufficient to remove strongly adhering particulate contamination.Furthermore, adding a solvent to the supercritical fluid and allowingthe wafer to soak may dissolve some contaminants but is not sufficientto dislodge strongly adhering particulate contamination. Therefore, evenwith supercritical fluid it is necessary to apply sufficient energy tothe particle/wafer interface to dislodge the particulate contamination.

A prior art approach for applying energy in the form of shear force tothe particle/wafer interface involves repeatedly filling and flushingthe chamber with supercritical fluid. The flow of supercritical fluidover the wafer surface during the flush is intended to impart sufficientshear force to the particle/wafer interface to dislodge the particulatecontamination. As particles decrease in size, the linear velocity of thesupercritical fluid required to dislodge the particles increases. Forexample, a supercritical fluid linear velocity of about 100 cm/sec isrequired to dislodge a particle having a size of about 0.1 micron.Unfortunately, using the fill and flush approach to obtain sufficientsupercritical fluid linear velocities at the particle/wafer interface todislodge smaller particles is difficult, if not impossible. One reasonfor this is that it is not reasonable to design and operate a chamberwhich relies on the flushing to impart the necessary linear velocity tothe supercritical fluid to cause particulate contamination to bedislodged from the wafer surface. To cause the supercritical fluid toflow at the required velocity during the flush operation, a sufficientpressure drop must exist across the chamber. However, both a pressuregreater than P_(c) and a temperature greater than T_(c) must bemaintained within each region of the chamber during the flush operationto preserve the supercritical phase of the supercritical fluid.Additionally, for the larger particulate contamination which may beremoved with the fill and flush approach, multiple fill and flush cyclesare required to adequately remove the particulate contamination. Use ofmultiple fill and flush cycles is time-consuming and not suitable forsingle-wafer process cycle times.

In view of the foregoing, there is a need for an apparatus and a methodfor effectively and efficiently using supercritical fluid to removeparticulate contamination from a wafer surface. The apparatus and methodshould avoid the problems associated with relying on high supercriticalfluid linear velocities to remove small particulate contamination. Theapparatus and method should also avoid the use of extended single-waferprocess cycle times.

SUMMARY OF THE INVENTION

Broadly speaking, the present invention fills these needs by providingan apparatus and a method for using high-frequency acoustic energy witha supercritical fluid to perform a semiconductor wafer (“wafer” or“substrate”) cleaning process. More specifically, the present inventionapplies high-frequency acoustic energy to the supercritical fluid toimpart energy to particulate contamination present on the wafer surface.Energy imparted to particulate contamination via the high-frequencyacoustic energy and supercritical fluid is used to overcomeintermolecular forces acting to adhere the particulate contamination tothe wafer. Additionally, the wafer cleaning process afforded by thepresent invention is enhanced by the beneficial properties of thesupercritical fluid. It should be appreciated that the present inventioncan be implemented in numerous ways, including as a process, anapparatus, a system, a device, or a method. Several embodiments of thepresent invention are described below.

In one embodiment, an apparatus for cleaning a wafer is provided. Theapparatus includes a volume configured to contain a supercritical fluid.Within the volume, the supercritical fluid is in contact with a wafer.Additionally, a number of transducers are in communication with thevolume. The number of transducers are capable of applying acousticenergy to the supercritical fluid contained within the volume. Theacoustic energy applied to the supercritical fluid is capable ofimparting energy to particulate contamination present on the wafersurface. The energy imparted to the particulate contamination present onthe wafer surface causes the particulate contamination to be dislodgedfrom the wafer surface, thus cleaning the wafer.

In another embodiment, another apparatus for cleaning a wafer isprovided. The apparatus includes a chamber having an internal volume.The chamber is configured to maintain a supercritical fluid within theinternal volume. The apparatus also includes one or more transducersdisposed in communication with the internal volume. The transducers arecapable of applying acoustic energy to the supercritical fluid to bemaintained within the internal volume of the chamber. When a wafer ispresent in the chamber internal volume, the acoustic energy applied tothe supercritical fluid is capable of imparting energy to particulatecontamination present on the wafer surface. The energy imparted to theparticulate contamination present on the wafer surface causes theparticulate contamination to be dislodged from the wafer surface, thuscleaning the wafer.

In another embodiment, a method for cleaning a wafer is provided. Themethod includes exposing a wafer to a supercritical fluid and applyingacoustic energy to the supercritical fluid. The acoustic energy appliedto the supercritical fluid is capable of imparting energy to particulatecontamination present on the wafer. The energy imparted to theparticulate contamination present on the wafer causes the particulatecontamination to be removed from the wafer, thus cleaning the wafer.

In another embodiment, another method is disclosed for cleaning a wafer.The method includes disposing a wafer between a first plurality oftransducers and a second plurality of transducers. The first pluralityof transducers includes transducers of different shape. The secondplurality of transducers mirrors the first plurality of transducers withregard to a shape of each transducer. The method also includes exposingthe wafer to a supercritical fluid. The method further includesoperating the first and second plurality of transducers to generate andtransfer acoustic energy through the supercritical fluid to the wafer.

In another embodiment, another method is disclosed for cleaning a wafer.The method includes disposing a wafer between a first plurality oftransducers and a second plurality of transducers. The second pluralityof transducers mirrors the first plurality of transducers with regard toa geometric configuration of each transducer. The method also includesexposing the wafer to a supercritical fluid. The method further includesactivating a pair of mirrored transducers so as to generate and transferacoustic energy through the supercritical fluid to the wafer in adirection extending between the pair of mirrored transducers. The pairof mirrored transducers is defined by one transducer in the firstplurality of transducers and one transducer in the second plurality oftransducers.

In another embodiment, another method is disclosed for cleaning a wafer.The method includes disposing a wafer between a first plurality oftransducers and a second plurality of transducers. Each of the firstplurality of transducers is mirrored by a transducer in the secondplurality of transducers to form a mirrored pair of transducers. Themethod also includes exposing the wafer to a supercritical fluid. Themethod further includes cyclically activating each mirrored pair oftransducers so as to transfer acoustic energy through the supercriticalfluid to different areas the wafer. Each of the different areas of thewafer corresponds to activation of a different mirrored pair oftransducers.

Other aspects and advantages of the invention will become more apparentfrom the following detailed description, taken in conjunction with theaccompanying drawings, illustrating by way of example the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further advantages thereof, may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings in which:

FIG. 1A is an illustration showing the collection and adherence of anaqueous or semi-aqueous solution between high aspect ratio waferstructures present on the surface of a wafer following a wet-cleaningprocess, in accordance with the prior art;

FIG. 1B is an illustration showing distortion damage of the structurescaused by capillary force collapse during the drying process, inaccordance with the prior art;

FIG. 1C is an illustration showing delamination damage of the structurescaused by capillary force collapse during the drying process, inaccordance with the prior art;

FIG. 2 is an illustration showing a generalized material phase diagram,in accordance with the prior art;

FIGS. 3A-3C are illustrations showing a chamber for performing a wafercleaning operation using high-frequency acoustic energy applied to asupercritical fluid, in accordance with one embodiment of the presentinvention;

FIGS. 4A-4C are illustrations showing the chamber incorporating amulti-level transducer arrangement, in accordance with one embodiment ofthe present invention; and

FIG. 5 is an illustration showing a flowchart of a method for usinghigh-frequency acoustic energy with a supercritical fluid to perform awafer cleaning process, in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An invention is disclosed for an apparatus and a method for usinghigh-frequency acoustic energy with a supercritical fluid to perform asemiconductor wafer (“wafer” or “substrate”) cleaning process. Thepresent invention applies high-frequency acoustic energy to thesupercritical fluid to impart energy to particulate contaminationpresent on the wafer surface. Energy imparted to particulatecontamination via the high-frequency acoustic energy and supercriticalfluid is used to overcome intermolecular forces acting to adhere theparticulate contamination to the wafer. Additionally, the wafer cleaningprocess afforded by the present invention benefits from thesupercritical fluid properties of near zero surface tension, highdiffusivity, high density, and chemical mixing capability.

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one skilled in the art that the presentinvention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail in order not to unnecessarily obscure the presentinvention.

FIGS. 3A-3C are illustrations showing a chamber 301 for performing awafer cleaning operation using high-frequency acoustic energy applied toa supercritical fluid, in accordance with one embodiment of the presentinvention. The chamber 301 cross-sections illustrated in FIGS. 3A and 3Bare identified in FIG. 3C. The chamber 301 includes top, bottom, andside walls surrounding an internal volume 303. In one embodiment, theperiphery of the chamber 301 is cylindrical with horizontal top andbottom members. However, in other embodiments the chamber 301 may have apolygonal peripheral shape such as rectangular, pentagonal, hexagonal,etc. Additionally, in other embodiments the chamber 301 top and bottommembers may be hemispherical or polygonal in shape. Regardless of theirspecific shape, the chamber 301 walls are capable of maintaining apressure within the internal volume 303 of up to about 200 atm. As usedherein, the term “about” means within ±10% of a specified value. Thechamber 301 is also equipped with temperature monitoring and controldevices to maintain a necessary temperature within the internal volume303 to ensure the integrity of a supercritical fluid state. Withreference to FIG. 3B, the chamber 301 is configured with inlets andoutlets through which the supercritical fluid is introduced and removedas indicated by arrows 305 and 307. In other embodiments the chamber 301can be equipped with a number of inlets and outlets depending on thedesired flow control to be applied to the supercritical fluid.Additionally, inlets and outlets can be configured radially and/orcentrally, whereby fluids are injected from the periphery and exitthrough the top and/or bottom center of the chamber. Additionally, thefluids can be injected through the top and/or bottom center of thechamber, and exit from the periphery.

The internal volume 303 is also configured to receive a wafer forcleaning. In one embodiment, the supercritical fluid is applied to allsurfaces of the wafer. In other embodiments, the supercritical fluid canbe applied to specific surfaces of the wafer. For example, if the waferis positioned to contact a support structure within the internal volume303, the surfaces of the wafer in contact with the support structure maynot be exposed to the supercritical fluid. In one embodiment, theinternal volume 303 is sized such that the wafer, when disposed withinthe internal volume 303, will be surrounded by a supercritical fluidthickness up to about 0.5 inch on each side. In one embodiment, thesupercritical fluid thickness can be different for each side of thewafer. Furthermore, in other embodiments, the supercritical fluidthickness can be greater than 0.5 inch for particular sides. In general,the internal volume 303 is sized to allow high-frequency acoustic energyto achieve optimal activation of the supercritical fluid at the wafersurface.

A number of transducers (T1, T2, and T3) are disposed around theinternal volume 303. Transducers positioned at the top, bottom, andsides of the internal volume 303 are secured to resonators R1, R2, andR3 respectively. The transducers and resonators are configured to applyhigh-frequency acoustic energy to the supercritical fluid to becontained and maintained within the internal volume 303. The transducerscan be composed of piezoelectric material such as piezoelectric ceramic,lead zirconium tintanate, piezoelectric quartz, or gallium phosphate,among others. The resonators are essentially plates that can be composedof ceramic, silicon carbide, stainless steel, aluminum, or quartz, amongothers.

The transducer configurations on the top and periphery of the chamber301 are shown in FIG. 3C. The transducer configurations on the bottom ofthe chamber 301 match the transducer configurations on the top of thechamber 301 (i.e., the top and bottom transducers are mirrored withrespect to each other). To facilitate impedance matching, each of thetransducers T1 and T2 distributed across the top and bottom of thechamber 301 should have approximately the same surface area facing thechamber internal volume 303. Activation of the various transducers canbe cycled or alternated such that only a portion of the transducers areon at a given time. The top and bottom transducers can be used toactivate the supercritical fluid in a direction perpendicular to thewafer surface. The side transducers can be used to activate thesupercritical fluid in a direction across the wafer surface. Thetransducers can also be attached to the resonator in a variety of ways.For example, the transducers can be attached flush to their respectiveresonator to be either horizontal, perpendicular, or angled to the wafersurface. Attachment of the transducers at an angle to the wafer surfacecan be used to improve acoustic energy stability by reducing impedancevariation and harmonic interference caused by acoustic energy reflectionfrom the wafer surface. In one embodiment, the transducers can be angledbetween about 2° and about 10° relative to a vector perpendicular to thewafer surface.

In the embodiment of FIGS. 3A-3C, the transducers on the top and bottomof the chamber are arranged in a single level, respectively. Thus, eachof the transducers T1 and T2 must be separated by a gap to avoidphysical contact and corresponding damage during activation. In oneembodiment, the gap is about 1 mm. Since the acoustic energy generatedby the transducers is transmitted in a substantially collimated mannertoward the wafer surface, it is desirable to minimize the gap betweenadjacent transducers to obtain a corresponding minimization of wafersurface area to which acoustic energy is not applied. To eliminateeffects of the gap (i.e., energy dead-zone), the wafer can be rotated soas to ensure all regions of the wafer are sufficiently covered. Rotatingthe wafer in a supercritical fluid process chamber is difficult and addsa significant amount of complexity and cost to the design. As analternative to the single level transducer arrangement requiring thepresence of gaps between adjacent transducers or a means of rotating thewafer, a multi-level transducer arrangement can be used to eliminate theneed for gaps between adjacent transducers.

FIGS. 4A-4C are illustrations showing the chamber 301 incorporating amulti-level transducer arrangement, in accordance with one embodiment ofthe present invention. FIGS. 4A-4C are analogous to FIGS. 3A-3C,respectively, with the exception that the top and bottom transducers arearranged in multiple levels, respectively. By arranging the transducersT1 and T2 in multiple levels, the edge of a transducer on one level canbe positioned equal with an edge of an adjacent transducer on anotherlevel. Therefore, gaps between adjacent transducers are not necessaryand can be eliminated. Elimination of gaps between adjacent transducersallows acoustic energy to be applied to the entire wafer surface.

High-frequency acoustic energy imparted from the transducers through theresonators to the supercritical fluid causes acoustic streaming of thesupercritical fluid which results in an enhancement of mass transportand increased physical interaction between the supercritical fluid andthe wafer surface. Thus, the high-frequency acoustic energy impartedthrough the supercritical fluid is used to transfer energy to theparticle-substrate interface and break the attractive forces between theparticle and wafer surface, facilitating removal of the contaminationfrom the wafer surface by the acoustic streaming phenomena. Therefore,rather than relying solely on shear force provided by the supercriticalfluid linear velocity, the present invention provides for the use ofhigh-frequency acoustic energy to mechanically dislodge particulatecontamination from the wafer surface and aid removal by acousticstreaming. The high-frequency acoustic energy can be applied to bothnon-flowing and flowing supercritical fluid. Thus, by applyinghigh-frequency acoustic energy to the supercritical fluid it is possibleto enhance particle de-adhesion from the wafer surface, reduce shearforce required to remove particles from the wafer surface, and reducecleaning process cycle times.

Through the use of high-frequency acoustic energy as afforded by thepresent invention, the beneficial properties of supercritical fluid canbe exploited in a practical and efficient wafer cleaning process. Thenear zero surface tension of supercritical fluid allows thesupercritical fluid to get into small features on the wafer surface.Furthermore, the near zero surface tension of the supercritical fluidallows for its easy removal from high aspect ratio features on the waferduring the drying process, as the pressure can be lowered to convert thefluid from a supercritical phase to a gas phase. Additionally, thesupercritical fluid has a diffusivity property similar to that of a gas.Therefore, the supercritical fluid can get into and out of porousregions of wafer materials, such as low-K dielectric material, withrelative ease. The supercritical fluid also has a density comparable tothat of a liquid. Hence, the supercritical fluid is capable ofefficiently transmitting the high-frequency acoustic energy generated bythe transducers. Furthermore, due to its relatively high density, asignificant amount of supercritical fluid can be transported to thewafer in a given amount of time. Transport of the supercritical fluid tothe wafer is important not only from a mechanical perspective, but alsofrom a chemical reaction perspective.

The supercritical fluid can be used as a transport mechanism for smalleramounts of chemicals that can enhance reactivity or removal of certaintarget contaminants. For example, the supercritical fluid canincorporate additives such as co-solvents and co-chelating agents.Examples of co-solvents and co-chelating agents that are compatible withsupercritical fluid include TiCl₄, SF₆, BF₃, C₂F₆,1,1,1,5,5,5,-hexafluoro-2,4-pentanedione,bis(trifluoroethyldithiocarbamate), metholheptafluorobutyrichydroxamicacid, beta-diketonates including crown ethers, dithiocarbamates, amines,hydroxamic acids and organophosphates. Also, the supercritical fluid canincorporate surfactant additives to discourage re-deposition of removedparticles back onto the wafer. In addition to accommodating additives,the supercritical fluid itself can be composed of either one material ora mixture of materials. Examples of materials that can be used forsupercritical fluid include CO₂, H₂O, Ne, N₂, Ar, Xe, SF₆, C₃H₈, andNH₃, among others.

FIG. 5 is an illustration showing a flowchart of a method for usinghigh-frequency acoustic energy with a supercritical fluid to perform awafer cleaning process, in accordance with one embodiment of the presentinvention. The method begins with an operation 501 in which a wafer isloaded into a chamber. The chamber is configured to maintain asupercritical fluid state and apply high-frequency acoustic energy tothe supercritical fluid. The method continues with an operation 503 inwhich the chamber is pressurized and the temperature within the chamberis controlled. The chamber pressure and temperature are controlled tomaintain a supercritical fluid state. In an exemplary embodiment, thechamber can be pre-pressurized with CO₂ only or with a mixture of CO₂and an appropriate chemistry. The critical pressure and temperature forCO₂ is approximately 73 atm and 31° C., respectively. It should be notedthat the method of FIG. 5 is not restricted to using CO₂. Other suitablesupercritical fluids can also be used.

In an operation 505, the supercritical fluid having an appropriatechemistry is introduced into the chamber. The supercritical fluid can beintroduced to only fill the chamber volume or to fill and provide acontinuous flow through the chamber volume. The chemistry of thesupercritical fluid may include additives such as co-solvents,co-chelating agents, surfactants, or any combination thereof. Theadditives contained within the supercritical fluid can be useful forperforming specific functions, such as dissolving and removingphotoresist, dissolving and removing organic residue, and chelatingmetals, among others.

The method continues with an operation 507 in which high-frequencyacoustic energy is applied to the supercritical fluid contained withinthe chamber. The high-frequency acoustic energy is applied to the waferfor an amount of time sufficient to obtain adequate cleaning results. Inone embodiment, the high-frequency acoustic energy application time fora single wafer can be within a range extending from about 5 seconds toabout 120 seconds. In another embodiment, the high-frequency acousticenergy application time for a single wafer can be within a rangeextending from about 15 seconds to about 60 seconds, with an averageapplication time of about 30 seconds. In a preferred embodiment, thehigh-frequency energy application time is minimized to optimize waferthroughput. In one embodiment, the high-frequency acoustic energy can beapplied with a frequency within a range extending from about 200 kHz toabout 50 MHz. In another embodiment, the high-frequency acoustic energycan be applied with a frequency within a range extending from about 500kHz to about 2 MHz. The high-frequency acoustic energy can also beapplied directionally to the supercritical fluid and ultimately thewafer by activating certain combinations of transducers at a given time.For example, the top and bottom transducers can be used to activate thesupercritical fluid in a direction perpendicular to the wafer surface.Alternatively, the side transducers can be used to activate thesupercritical fluid across the wafer surface. Also, the high-frequencyacoustic energy can be applied from different combinations oftransducers on a cyclic basis.

After application of the high-frequency acoustic energy, the methodcontinues with an operation 509 in which the supercritical fluid ispurged from the chamber. The supercritical fluid purge generallyincludes de-pressurization and flushing of the chamber. In oneembodiment, a CO₂ only rinse may be performed as part of operation 509prior to depressurization of the chamber. The CO₂ only rinse can be usedto ensure that chemical additives and removed contaminants are no longerpresent in the chamber.

The method then proceeds to a decision operation 511 in which adetermination is made to either repeat the cleaning process using adifferent supercritical fluid chemistry or terminate the cleaningprocess. In some wafer cleaning operations, different types ofcontaminants may require the use of a specialized chemistry. Thus, theoption to repeat the cleaning process using a different supercriticalfluid chemistry provides for sequential targeting of particular types ofcontaminants. For example, in one pass through operations 503-509 asupercritical fluid containing co-solvent A may be used to removeparticle X. Then in a subsequent pass through operations 503-509 asupercritical fluid containing co-solvent B may be used to removeparticle Y, etc. Use of specialized supercritical fluid chemistries insequential passes through operations 503-509 can be used to dissolveparticular materials, chelate particular materials, and preventre-deposition of particular materials. It is therefore apparent that themethod of FIG. 5 offers significant flexibility in defining a wafercleaning process that is tailored for a particular wafer condition.

Application of high-frequency acoustic energy as provided by the presentinvention allows the beneficial properties of supercritical fluid to beexploited in a highly practical and efficient wafer cleaning process. Byapplying high-frequency acoustic energy to the supercritical fluid, theunreasonable reliance on high linear velocities of the supercriticalfluid to mechanically dislodge particulates from the wafer surface iseliminated. The high-frequency acoustic energy applied to thesupercritical fluid is transferred to the particles such that the strongintermolecular forces binding the particles to the wafer are overcome.With the present invention the supercritical fluid properties of nearzero surface tension, high diffusivity, high density, and chemicalmixing capability can be used to efficiently and effectively perform awafer cleaning process.

While this invention has been described in terms of several embodiments,it will be appreciated that those skilled in the art upon reading thepreceding specifications and studying the drawings will realize variousalterations, additions, permutations and equivalents thereof. It istherefore intended that the present invention includes all suchalterations, additions, permutations, and equivalents as fall within thetrue spirit and scope of the invention.

1. A method for cleaning a wafer, comprising: disposing a wafer betweena first plurality of transducers and a second plurality of transducerssuch that the first plurality of transducers are positioned above a topsurface of the wafer and such that the second plurality of transducersare positioned below a bottom surface of the wafer, wherein the firstplurality of transducers positioned above the top surface of the waferincludes transducers of different shape that each have a same surfacearea size facing the top surface of the wafer, and wherein the secondplurality of transducers positioned below the bottom surface of thewafer mirrors the first plurality of transducers positioned above thetop surface of the wafer with regard to a shape and a size of eachtransducer, such that each transducer in both the first and secondpluralities of transducers is defined to have approximately a samesurface area size facing the wafer; exposing the wafer to asupercritical fluid; and operating the first and second plurality oftransducers to generate and transfer acoustic energy through thesupercritical fluid to the wafer.
 2. A method for cleaning a wafer asrecited in claim 1, wherein the wafer is disposed such that a topsurface of the wafer is substantially parallel to a surface of each ofthe first plurality of transducers, and a bottom surface of the wafer issubstantially parallel to a surface of each of the second plurality oftransducers.
 3. A method for cleaning a wafer as recited in claim 1,further comprising: maintaining a temperature within the chamber at orabove a critical temperature of the supercritical fluid; and maintaininga pressure within the chamber at or above the critical pressure of thesupercritical fluid.
 4. A method for cleaning a wafer as recited inclaim 1, further comprising: flowing the supercritical fluid over a topsurface of the wafer.
 5. A method for cleaning a wafer as recited inclaim 1, wherein the first and second plurality of transducers areoperated to generate acoustic energy having a frequency within a rangeextending from about 200 kHz to about 50 MHz.
 6. A method for cleaning awafer as recited in claim 1, wherein the acoustic energy is generatedfor a duration extending from about 5 seconds to about 120 seconds.
 7. Amethod for cleaning a wafer as recited in claim 1, wherein thesupercritical fluid includes one or more of a co-solvent, a co-chelatingagent, and a surfactant.
 8. A method for cleaning a wafer, comprising:disposing a wafer between a first plurality of transducers and a secondplurality of transducers such that the first plurality of transducersare positioned above a top surface of the wafer and such that the secondplurality of transducers are positioned below a bottom surface of thewafer, wherein the second plurality of transducers mirrors the firstplurality of transducers with regard to a geometric configuration ofeach transducer, and wherein the first plurality of transducers isdefined in multiple vertically separated levels such that adjacenttransducers within the first plurality of transducers are disposed indifferent ones of the multiple vertically separated levels so as toavoid gaps between adjacent transducers within the first plurality oftransducers; exposing the wafer to a supercritical fluid; and activatinga pair of mirrored transducers defined by one transducer in the firstplurality of transducers and one transducer in the second plurality oftransducers so as to generate and transfer acoustic energy through thesupercritical fluid to the wafer in a direction extending between thepair of mirrored transducers.
 9. A method for cleaning a wafer asrecited in claim 8, wherein the direction extending between the pair ofmirrored transducers is substantially perpendicular to a top surface ofthe wafer.
 10. A method for cleaning a wafer as recited in claim 8,wherein the wafer is disposed such that a top surface of the wafer issubstantially parallel to a surface of each of the first plurality oftransducers, and a bottom surface of the wafer is substantially parallelto a surface of each of the second plurality of transducers.
 11. Amethod for cleaning a wafer as recited in claim 8, wherein activatingthe pair of mirrored transducers is performed in a cyclical manner suchthat different pairs of mirrored transducers in the first and secondpluralities of transducers are activated at respectively differenttimes.
 12. A method for cleaning a wafer as recited in claim 8, furthercomprising: activating one or more transducers around a periphery of thewafer to generate and transfer acoustic energy through the supercriticalfluid in a direction substantially parallel to a top surface of thewafer and in proximity to the top surface of the wafer.
 13. A method forcleaning a wafer as recited in claim 8, further comprising: exposing thewafer to a clean supercritical fluid rinse, the clean supercriticalfluid rinse comprising supercritical fluid without additives.
 14. Amethod for cleaning a wafer as recited in claim 13, further comprising:exposing the wafer to a second supercritical fluid, wherein the secondsupercritical fluid has a different chemistry than a previoussupercritical fluid; and applying acoustic energy to the secondsupercritical fluid.
 15. A method for cleaning a wafer, comprising:disposing a wafer between a first plurality of transducers and a secondplurality of transducers such that the first plurality of transducersare positioned above a top surface of the wafer and such that the secondplurality of transducers are positioned below a bottom surface of thewafer, wherein each of the first plurality of transducers is mirrored bya transducer in the second plurality of transducers to form a mirroredpair of transducers, wherein each transducer in both the first andsecond pluralities of transducers is defined to have a same surface areasize facing the wafer, wherein the first plurality of transducers isdefined in multiple vertically separated levels above the top surface ofthe wafer such that adjacent transducers within the first plurality oftransducers are disposed in different ones of the multiple verticallyseparated levels above the top surface of the wafer so as to avoid gapsbetween adjacent transducers within the first plurality of transducers,and wherein the second plurality of transducers is defined in multiplevertically separated levels below the bottom surface of the wafer suchthat adjacent transducers within the second plurality of transducers aredisposed in different ones of the multiple vertically separated levelsbelow the bottom surface of the wafer so as to avoid gaps betweenadjacent transducers within the second plurality of transducers;exposing the wafer to a supercritical fluid; and cyclically activatingeach mirrored pair of transducers so as to transfer acoustic energythrough the supercritical fluid to different areas the wafer, whereineach of the different areas of the wafer corresponds to activation of adifferent mirrored pair of transducers.
 16. A method for cleaning awafer as recited in claim 15, wherein the cyclically activating isperformed to ensure that an entirety of a top surface of the wafer issubjected to acoustic energy.
 17. A method for cleaning a wafer asrecited in claim 15, wherein the wafer is disposed such that a topsurface of the wafer is substantially parallel to a surface of each ofthe first plurality of transducers, and a bottom surface of the wafer issubstantially parallel to a surface of each of the second plurality oftransducers.
 18. A method for cleaning a wafer as recited in claim 15,further comprising: activating one or more transducers around aperiphery of the wafer to generate and transfer acoustic energy throughthe supercritical fluid in a direction substantially parallel to a topsurface of the wafer and in proximity to the top surface of the wafer.19. A method for cleaning a wafer as recited in claim 18, whereinactivation of one or more transducers around the periphery of the waferis performed between cyclic activation of each mirrored pair oftransducers.
 20. A method for cleaning a wafer as recited in claim 18,wherein activation of one or more transducers around the periphery ofthe wafer is performed simultaneously with activation of each mirroredpair of transducers.